Method of Fabricating a Flexible Photovoltaic Film Cell With an Iron Diffusion Barrier Layer

ABSTRACT

A method of fabricating a flexible photovoltaic film cell with an iron diffusion barrier layer. The method includes: providing a foil substrate including iron; forming an iron diffusion barrier layer on the foil substrate, where the iron diffusion barrier layer prevents the iron from diffusing; forming an electrode layer on the iron diffusion barrier layer; and forming at least one light absorber layer on the electrode layer. A flexible photovoltaic film cell is also provided, which cell includes: a foil substrate including iron; an iron diffusion barrier layer formed on the foil substrate to prevent the iron from diffusing; an electrode layer formed on the iron diffusion barrier layer; and at least one light absorber layer formed on the electrode layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/245,016, filed Sep. 26, 2011, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to photovoltaic devices, and moreparticularly the present invention is related to a flexible photovoltaicfilm cell with an iron diffusion barrier layer.

Today, Stainless steel foils and copper foils have been identified aspotential flexible photovoltaic substrates for CIGS/CZTS. However, thepast flexible photovoltaic film cells have significantly lower lightconversion efficiency, because iron in stainless steel tends to diffuseup into the light absorber layer and interfere with the light absorberlayer.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the present invention provides a method offabricating a flexible photovoltaic film cell with an iron diffusionbarrier layer, the method including the steps of: providing a foilsubstrate including iron; forming an iron diffusion barrier layer on thefoil substrate, where the iron diffusion barrier layer prevents the ironfrom diffusing; forming an electrode layer on the iron diffusion barrierlayer; and forming at least one light absorber layer on the electrodelayer.

Another aspect of the present invention provides a flexible photovoltaicfilm cell, including: a foil substrate comprising iron; an irondiffusion barrier layer formed on the foil substrate, where the irondiffusion barrier layer prevents the iron from diffusing; an electrodelayer formed on the iron diffusion barrier layer; and at least one lightabsorber layer formed on the electrode layer.

The foregoing features are of representative embodiments and arepresented to assist in understanding the invention. It should beunderstood that they are not intended to be considered limitations onthe invention as defined by the claims, or limitations on equivalents tothe claims. Therefore, this summary of features should not be considereddispositive in determining equivalents. Additional features of theinvention will become apparent in the following description, from thedrawings and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a flexible photovoltaic film cell with aniron diffusion barrier layer according to an embodiment of theinvention.

FIG. 2 shows a schematic diagram of a method for producing a flexiblephotovoltaic film cell with an iron diffusion barrier layer according toan embodiment of the invention.

FIG. 3 shows a cross section image of a flexible photovoltaic film cellwith an iron diffusion barrier layer under scanning electron microscopeaccording to an embodiment of the invention.

FIG. 4A shows a depth profiling graph of iron in a flexible photovoltaicfilm cell with an iron diffusion barrier layer during an annealingprocess according to an embodiment of the invention.

FIG. 4B shows a depth profiling graph of iron in a flexible photovoltaicfilm cell without an iron diffusion barrier layer during an annealingprocess according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above and other features of the present invention will become moredistinct by a detailed description of embodiments shown in combinationwith attached drawings. Identical reference numbers represent the sameor similar parts in the attached drawings of the invention.

Flexible photovoltaics (FPV) have many advantages over conventionalPhotovoltaics (PV) which are fabricated on rigid, heavy, and fragileglass substrates. FPVs use thin metal foils which is light but morerobust—only a few kilogram for solar panels of tens of square meters,and hard to break. Furthermore, because of their flexibility, they canbe not only incorporated onto building sectors, but also on automobilesectors and consumable electronics sectors, therefore open up avenue fornew applications of PV devices.

Stainless steel foils and copper foils have been identified as potentialFPV substrates for CIGS/CZTS light absorber layers, because stainlesssteel foils and copper foils have matching thermal expansioncoefficients to CIGS/CZTS and good corrosion resistance. However, ironin stainless steel tends to diffuse up into the light absorber layer,dramatically lowering the light conversion efficiency. In addition,copper up-diffusion can also change the chemical composition ofCIGS/CZTS, resulting in rapid aging of solar devices. The currentsolution to prevent the diffusion of detrimental elements from substrateis to coat the metal foil with a barrier layer. The barrier layermaterials adopted so far include refractory metals and oxides such asTi, Ta, and SiO2, but these refractory materials have limited effect inpreventing iron from diffusing up into the light absorber layer.

A detailed description of a method and apparatus for fabricating aflexible photovoltaic film cell as provided by an embodiment of thepresent invention is made with reference to attached drawings.

FIG. 1 illustrates a flexible photovoltaic film cell that forms aflexible photovoltaic device 100 including an iron diffusion barrierlayer, according to an embodiment of the invention. For example, thesolar cell 100 can be formed according to method shown in FIG. 2. Thethin film solar cell 100 also includes Copper indium gallium di-selenide(CIGS), which is a semiconductor light absorbing material having adirect bandgap. The term CIGS (CuInxGa(1-x)Se2), which will be explainedin detail later, is a compound included of copper, indium, and either orboth of gallium and selenium. In the broad sense, at one extreme CIGScan be the compound CIS that does not include gallium (X=1); at theother extreme CIGS can be the compound CGS that does not include indium(X=0); or CIGS can be a compound containing all of the elements: copper,indium, gallium and selenium (X is between 0 and 1, but not including 0and 1). Also, CIGS (CuInxGa(1-x)Se2) has a bandgap varying continuouslywith X from about 1.0 eV (electron volts) at 300 K (degrees Kelvin) forCIS (X=1), to about 1.7 eV at 300 K for CGS (X=0).

The flexible photovoltaic film cell includes a substrate 110, an irondiffusion barrier layer 120, an electrode layer 130, and a lightabsorber layer 140. The substrate 110 is an iron foil layer upon orabove which the other layers of the cell 100 are formed. The iron foilsubstrate can provide mechanical support for cell 100. In addition tothe mechanical support, an iron foil substrate can provide corrosionresistance, flexibility, lower production costs and durability. The ironfoil substrate can have a thickness of about 0.14 millimeters (mm).Other exemplary substrates include stainless steel foil and other foilsubstrates.

Iron diffusion barrier layer 120 is a layer formed on the foil substrate110. Iron diffusion barrier layer 120 includes a material that hasmetallic species and a non-metallic species, and can includenickel-phosphorus (NiP). Iron diffusion barrier layer in the embodimentof the present invention has a thickness from 50 nm to 1000 nm,depending on the barrier diffusion properties, which can be affected bychemical composition of phospher in NiP barrier layer, as well as theprocessing temperature.

A back contact electrode layer 130 is a layer formed upon the irondiffusion barrier layer that was annealed on the substrate 110. The backelectrode layer 130 typically includes a metal and can include, forexample, molybdenum (Mo). Alternately or additionally, the back contact130 can include a semiconductor. The back contact 130 is an electricalcontact that provides back-side electrical contact to provide currentfrom the flexible photovoltaic film cell 100. An Exemplary back contact130 is a layer having a thickness from about 0.5 micron to about 1micron.

Light absorber layer 140 includes CIGS, and can be, for example, about 1to about 2 microns thick. The CIGS included within light absorber layer140 can be, for example, nanocrystalline (microcrystalline) orpolycrystalline and can be formed p-type, for example, formed p-typefrom intrinsic defects within the CIGS. Nanocrystalline andpolycrystalline CIGS both include crystalline grains, but differ in, forexample, the grain size of the crystalline grains. Alternately oradditionally, the CIGS can be formed p-type by intentional inclusion(e.g., doping) of a p-type dopant (i.e., an additional materialintroduced into the CIGS in very small concentrations to make the CIGSsemiconductor p-type or more p-type). The light absorber layer 140 canalso include, alternatively, an approximately 0.7 microns thick layer ofn-type CdS. The light absorber layer 140 is formed upon and abuts or isproximate to the electrode layer 130.

FIG. 2 shows a schematic diagram of a method for producing a flexiblephotovoltaic film cell with an iron diffusion barrier layer according toan embodiment of the invention. In step 810 of FIG. 2, an iron foilsubstrate 110 is provided. Foil substrate 110 can contain iron only, oriron and chromium. In an embodiment of the present invention, foilsubstrate contains 5 atom percent to 30 atom percent chromium. 16.5%percent chromium stainless foil substrate can be preferred, because itscoefficient of thermal expansion (CTE) matches with those of preferredabsorber layer materials such as CIGS as well as superior corrosionresistance. Another steel foil, nickel chromium steel foil has higherCTE than chromium stainless foil. For example, high Chromium StainlessSteel (SS) foil consisting of 84% Fe and 16% Cr of thickness 0.14 mm canbe selected as the flexible substrate because its coefficient of thermalexpansion (CTE) is close to that of CIGS/CZTS. Foil substrate 110 can bemanufactured through cold forming by all standard process (bending,contour forming, drawing, flow turning etc.), and can be cleaned withorganic solvents in an ultrasonic bath to remove oil residues fromrolling.

In step S20 of FIG. 2, iron diffusion barrier layer 120 is formed on thefoil substrate. Iron diffusion barrier layer in the embodiment of thepresent invention has a thickness from 50 nm to 1000 nm, depending onthe barrier diffusion properties, which can be affected by chemicalcomposition of phospher in NiP barrier layer, as well as the processingtemperature. Iron diffusion barrier layer 120 includes a material thathas metallic species and a non-metallic species. The metallic speciesconsist of one or more elements in Group VIA including Mo and W, ingroup VIIIB including Ni, Co and Fe, as well as Zn, Sn and Sb. Thenon-metallic species consist of one element in B, P, S, Se and Te. To bemore specific, typical binary compounds include NiP/B, CoP/B, MoP/B,MoSe2 and MoS2, and typical ternary compounds includes NiMoP/B, CoMoP/B,NiWP/B and CoWP/B. The iron diffusion barrier layer can be formed byspinning, spraying, anodization, electroless deposition,electrodeposition, vacuum deposition, and vapor deposition. Irondiffusion barrier layer 120 can be annealed on the foil substrate at arange from 400° C. to 600° C. in nitrogen gas atmosphere to improve theadhesion of the iron diffusion barrier layer on the foil substrate.

Referring to step S20 of FIG. 2, barrier layer materials can beelectroplated in aqueous solution and demonstrate their barrier propertyon metal foils. Deposition of nickel-phosphorus (NiP) with anelectroless solution have some properties that are superior to those ofelectrolytically formed NiP, because the electrolessly deposited NiP canbe controlled to have specific phosphorus content. Electrolesslydeposited NiP is harder, has great corrosion resistance, and has uniformsurface coverage due to the control of the phosphorus. Furthermore,without the costly equipment and energy to sustain a vacuum environment,solution-based electroplating is considered as a low-cost method forthin-film fabrication. Thus, electroplated diffusion barrier solves theproblem of FPV without introducing significant extra cost compared tovacuum process, which leads to the high final cost of flexible solarpanels. (see Daly et al., “Electrochemical nickel-phosphorus alloyformation” International Materials Reviews 2003 Vol. 48 No. 5, pp.326-338).

NiP of thickness about 300 nm can be electrolessly formed on the SSusing commercial electroless plating bath (Technic EN 8200). A preferredamount percentage of P in the NiP barrier is around 12%. The depositionrate is approximately 200 nm per minute, resulting in less than twominutes processing time. The as-formed NiP film is then soft-annealed at200° C. for 30 minutes in nitrogen atmosphere to improve adhesion. Thetemperature of the annealing is preferred to be kept at 200° C., for 30minutes because there may be structural changes of NiP if it is annealedbeyond the preferred temperature and time. (see Paunovic et al.,“Electrical Resistance and Stress of Bilayer Co(P)/Cu and Ni(P)/Cu ThinFilms,” J. Electrochem. Soc., Vol. 140, No. 9, September 199). In theembodiment of the present invention, however, the NiP will survive hightemperatures due to NiP layer's thickness.

Referring again to step S20 of FIG. 2, an iron diffusion barrier layercan alternatively be formed by electrolytic pulse deposition from a bathcontaining 0.11 M NiSO4, 0.17M NaH2PO2, 0.12M sodium acetate and 0.3 mMSDS (C12H25O4S—Na) with pH=4.4. The pulse deposition consists of twotypes of pulses: one is at 50 mA/cm2, 500 Hz for 10 s, the other is at10 mA/cm2, 1 Hz for 10 minutes. This method leads to higher P content asshown in Table 1. NiP film with higher P content can suppress the Fediffusion more effectively.

TABLE 1 Chemical composition and thickness of pulse electroformed NiPthin film Sample [Ni] at. % [P] at. % t (Å) SS1110E 73.4 ± 2 26.6 ± 23188 ± 200 SS1110B 74.4 ± 2 25.6 ± 2 2328 ± 200

In step S30 of FIG. 2, a back contact electrode layer 130 is formed oniron diffusion barrier layer 120. Electrode layer 130 is a back contactlayer, and is 600 nm thick. Electrode layer 130 includes compounds suchas molybdenum. Molybdenum compound is preferred due to its widelyapplication as a back contact material in CIGS/CZTS solar cells, becauseit forms a good Ohmic contact with the absorber layer. Molybdenumelectrodes have low stress, high conductivity, and good adhesion to therear substrate. To provide this combination of features, oxygen isintroduced into the molybdenum electrode at the initial stage ofdeposition on the substrate. The application of the oxygen reduces theoverall stress of the rear electrode. (see U.S. Pat. No. 7,875,945 B2,published on Jan. 25, 2011) The electrode layer is sputtered on thediffusion barrier layer 120.

In step S40 of FIG. 2, a light absorber layer 140 is formed on electrodelayer 130. Light absorber layer 140. Light absorber layer uses thin-filmtechnology, which includes direct bandgap materials, such as amorphousSi, cadmium telluride (CdTe) and copper indium gallium selenide(CuInGaSe₂ also commonly abbreviated as “CIGS”). Direct bandgapabsorbers have strong light absorption at a thickness of only a fewmicrometers. Reduced thickness means reduced material and productioncosts.

A bandgap (also called an energy gap) of a material, is an energy rangeof the material where no electron states exist. For insulators andsemiconductors, the bandgap generally refers to the energy differencebetween the top of the valence band of the material and the bottom ofthe conduction band of the material. The bandgap is the amount of energyrequired to free an outer-shell electron from its orbit about thenucleus to a free state. Bandgaps are usually expressed in electronvolts.

Copper indium gallium di-selenide (CIGS) is an I-III-VI₂, compoundsemiconductor material (e.g., a p-type semiconductor material). CIGS isalso known as copper indium gallium selenide. In the broad sense, CIGS,as used herein, indicates a compound comprised of copper, indium, andeither or both of gallium and selenium. That is, CIGS may be thecompound CIS, the compound CGS or a compound containing all the elementscopper, indium, gallium and selenium. CIGS may be a solid solution ofthe constituent elements of CIGS. CIGS has a chemical formula ofCuIn_(x)Ga_((1-x))Se₂, where the value of X can vary from 1 to 0. CIGSis a tetrahedrally-bonded semiconductor, with a chalcopyrite crystalstructure, and a bandgap varying continuously with X from about 1.0 eV(electron volts) at 300 K (degrees Kelvin) for CIS, to about 1.7 eV at300 K for CGS.

Of the three above-mentioned thin-film materials, CIGS is a preferredmaterial for the light absorber layer 140. CIGS-based photovoltaicdevices with an efficiency of as high as 19.5 percent (%) have beendemonstrated (as compared with 16.5% and 12% efficiencies for CdTe andamorphous Si absorbers, respectively). In addition, in CIGS there is notoxic cadmium (Cd) involved as with CdTe, and there are no degradationissues as with amorphous Si. (See U.S. Pat. No. 7,838,403 B1, publishedon Nov. 23, 2010). Furthermore, It is preferred for light absorber layer140 to include copper indium gallium selenide (CIGS), copper zinc tinsulfide (CZTS), and/or Cadmium Sulfide (CdS), because their high lightabsorption coefficients and satisfactory long-term stability.

Very-high-efficiency CIGS absorber layers have been achieved usingvacuum-based deposition processes, such as the “three-stage process”adopted by the National Renewable Energy Lab (NREL) which is a vacuumco-evaporation process wherein individual metal sources of copper (Cu),indium (In), gallium (Ga) and selenium (Se) are evaporated toward aheated substrate. The carefully controlled metal fluxes deliver adesired amount of metals, which react at the substrate under anoverpressure of Se and form the CIGS compound. (See U.S. Pat. No.7,838,403 B1, published on Nov. 23, 2010). Light absorber layer 140 canbe annealed onto the electrode layer at a range of 400° C. and 600° C.

Some of the other solution-based approaches include solution-baseddeposition, electrodeposition, spray processes, doctor blading, ink jetprinting and spin-coating. Spray processes, in particular, offer highthroughput and high material utilization, and can be used to producelarge-area uniform thin films with good adhesion to the substrate.Deposition of chalcopyrite materials, such as, copper indium disulfide(CuInS₂) and copper indium diselenide (CuInSe₂), which is commonlyabbreviated as “CIS,” have been demonstrated using this method.

After other approaches, light absorber layer 140 can be annealed ontothe electrode layer at a range of 400° C. and 600° C.

FIG. 3 shows an image of a flexible photovoltaic film cell with an irondiffusion barrier layer under scanning electron microscope. Referring toFIG. 3, the stack of the films Cu (80 nm)/Mo (600 nm)/NiP (300 nm)/SS isclearly shown in the scanning electron microscope (SEM) cross-sectionimage. In the image, iron diffusion barrier layer is compact anduniform, conformally coating the Stainless Steel substrate. Thecross-section image of the embodiment of the present invention showsas-formed multi-layer structure, highlighting the continuous dense irondiffusion barrier layer between the back contact Mo layer and thestainless steel substrate.

FIG. 4A shows a depth profiling graph of iron in a flexible photovoltaicfilm cell with an iron diffusion barrier layer during an annealingprocess according to an embodiment of the invention. As can be seen fromFIG. 4A., there is no observable increase of Ni content in the whole Molayer after 30 minutes annealing at 600° C.

On the other hand, FIG. 4B shows a depth profiling graph of iron in aflexible photovoltaic film cell without an iron diffusion barrier layerduring an annealing process according to an embodiment of the invention.As can be seen from FIG. 4B, Ni contents significantly increase after 30minutes annealing at 600° C. across the whole Mo layer, especially nearthe Cu/Mo interface.

It is to be understood that the sequence between the process steps shownin the accompanying figures and described herein can differ depending onthe manner in which the present invention is used to create a finalproduct such as a photovoltaic thin film structure. Given the teachingsof the present invention, one of ordinary skill in the art will be ableto contemplate these and similar implementations or configurations ofthe present invention.

It should also be understood that the above description is onlyrepresentative of illustrative embodiments. For the convenience of thereader, the above description has focused on a representative sample ofpossible embodiments, a sample that is illustrative of the principles ofthe invention. The description has not attempted to exhaustivelyenumerate all possible variations. Thus, alternative embodiments thatare not presented herein regarding a specific portion of the inventionor further alternatives that may be available but are not describedherein should not be considered to be a disclaimer of those alternateembodiments. Other applications and embodiments can be implementedwithout departing from the spirit and scope of the present invention.

It is therefore intended that the present invention not be limited tothe specific embodiments described herein, because numerous permutationsand combinations of the above and implementations involvingnon-inventive substitutions for the above can be created, but theinvention is to be defined in accordance with the claims that follow. Itcan be appreciated that many such embodiments are within the literalscope of the appended claims and that others are equivalents thereto.

What is claimed is:
 1. A method of fabricating a flexible photovoltaicfilm cell with an iron diffusion barrier layer, the method comprisingthe steps of: providing a foil substrate comprising iron; forming aniron diffusion barrier layer on said foil substrate, wherein said irondiffusion barrier layer prevents said iron from diffusing to at leastone light absorber layer, and wherein the iron diffusion barrier layercomprises a chemical compound selected from a group consisting of:cobalt-phosphorus (CoP), molybdenum-phosphorus (MoP), nickel-boron(NiB), molybdenum-boron (MoB), molybdenum (IV) selenide (MoSe₂),molybdenum disulfide (MoS₂), nickel molybdenum phosphide (NiMoP), nickelmolybdenum boride (NiMoB), cobalt molybdenum phosphide (CoMoP), cobaltmolybdenum boride (CoMoB), nickel-tungsten-phosphorus (NiWP),nickel-tungsten-boron (NiWB), cobalt tungsten phosphide (CoWP), and anycombinations thereof; forming an electrode layer on said iron diffusionbarrier layer; and forming at least one light absorber layer on saidelectrode layer.
 2. The method according to claim 1, wherein said stepof forming an iron diffusion barrier layer comprises a techniqueselected from the group consisting of spinning, spraying, anodization,electroless deposition, electrodeposition, vacuum deposition, and vapordeposition.
 3. The method according to claim 1, wherein said irondiffusion barrier layer has a thickness from 50 nm to 1000 nm.
 4. Themethod according to claim 1, wherein said step of forming an irondiffusion barrier layer further comprises: annealing said iron diffusionbarrier layer onto said foil substrate at a range from 400° C. to 600°C., wherein said step of annealing on said diffusion layer improvesadhesion of said iron diffusion barrier layer on said foil substrate. 5.The method according to claim 1, wherein said step of forming at leastone light absorber layer on said electrode layer further comprises:annealing said light absorber layer onto said electrode layer at a rangefrom 400° C. to 600° C., wherein said step of annealing on said lightabsorber layer improves adhesion of said light absorber layer on saidelectrode layer.
 6. The method according to claim 1, wherein said foilsubstrate further comprises chromium, wherein said foil substratecontains 5 atom percent to 30 atom percent chromium.
 7. The methodaccording to claim 1, wherein said light absorber layer is a layerselected from a group consisting of copper indium gallium selenide(CIGS) and copper zinc tin sulfide (CZTS), Cadmium Sulfide (CdS), andcombinations thereof.
 8. The method according to claim 1, wherein saidelectrode layer comprises molybdenum.
 9. A flexible photovoltaic filmcell, comprising: a foil substrate comprising iron; an iron diffusionbarrier layer formed on said foil substrate, wherein said iron diffusionbarrier layer prevents said iron from diffusing to at least one lightabsorber layer, and wherein said iron diffusion barrier layer comprisesa chemical compound selected from a group consisting of:cobalt-phosphorus (CoP), molybdenum-phosphorus (MoP), nickel-boron(NiB), molybdenum-boron (MoB), molybdenum (IV) selenide (MoSe₂),molybdenum disulfide (MoS₂), nickel molybdenum phosphide (NiMoP), nickelmolybdenum boride (NiMoB), cobalt molybdenum phosphide (CoMoP), cobaltmolybdenum boride (CoMoB), nickel-tungsten-phosphorus (NiWP),nickel-tungsten-boron (NiWB), cobalt tungsten phosphide (CoWP), andcombinations thereof; an electrode layer formed on said iron diffusionbarrier layer; and at least one light absorber layer formed on saidelectrode layer.
 10. The film cell according to claim 9, wherein saidiron diffusion barrier layer is formed by a technique selected from thegroup consisting of spinning, spraying, anodization, electrolessdeposition, electrodeposition, vacuum deposition, and vapor deposition.11. The film cell according to claim 9, wherein said iron diffusionbarrier layer has a thickness from 50 nm to 1000 nm.
 12. The film cellaccording to claim 9, wherein said iron diffusion barrier layer isannealed onto said foil substrate at a range of 400° C. and 600° C. 13.The film cell according to claim 9, wherein said light absorber layer isannealed onto said electrode layer at a range of 400° C. and 600° C. 14.The film cell according to claim 9, wherein said foil substrate furthercomprises chromium, wherein said foil substrate contains 5 atom percentto 30 atom percent chromium.
 15. The film cell according to claim 9,wherein said light absorber layer is a layer selected from a groupconsisting of copper indium gallium selenide (CIGS), copper zinc tinsulfide (CZTS), Cadmium Sulfide (CdS), and combinations thereof.
 16. Thefilm cell according to claim 9, wherein said electrode layer comprisesmolybdenum.